Can I estimate the balance on an open delta transformer bank.

[brj1]

I am trying to balance an electric distrubution circuit. I have the estimated KVA for each transformer bank and what phase it is on. On a three phase symetrical bank I take the load and divide it evenly across each phase 33% on each phase to get the load on each phase. On a single phase transformer bank I put 100% of the load on that phase that it is on. What I am wondering is how can I estimate the load on each phase of an open delta transformer bank. A guy that does this at another company told me to estimate 80% load on the biggest transformer and 20% load on the little transformer to estimate the load on each phase.

Does anyone have any tips?

Thanks

 

[waross]

If the open delta is supplying only three phase loads, the transformers will be loaded evenly.
The other most common application is a combination of a normal single phase load and a small three phase load. In this case a teaser transformer is added to the transformer supplying the normal single phase load. The large transformer will supply the single phase and about 86% of the three phase apparent load. The small transformer will supply about 86% of the three phase apparent load. There may be a 5HP motor on a 25KVA teaser transformer, so estimating the loading on the small transformer in an unequal open delta bank without knowing the connected load is a guess and not even a good guess.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brj1]

What do you mean connected load? I know the kwh and calculated kva load on the transformers. Is that what you mean?

Also you said that both transformers supply 86% of the three phase load, is this a typo? If not could you please explain.

You said that this would not be a good guess, but that is all I want; a formula to give me a good guess if anyone has one.

Thanks.

 

[burnt2x]

A sketch could help others re-think what you meant! From what I have read on the posts/ reply, it seems that you're not really asking for formulas on "open-delta"! Just plain single-phase trafos!
Hope I'm right but we can't answer completely with incomplete facts/ data.

 

[waross]

Consider two 25 KVA, 240V transformers in open delta.
The nominal capacity is 50 KVA. The rated transformer current is 104 amps. With an open delta the transformer must carry the entire line current rather than 86% of line current. So two 25 KVA transformers in open delta have a combined capacity of 104A x 240V x sqrt 3 = 43.3KVA.
Work some sample problems with different loads and currents, remebering that the load current equals the transformer current and I hope you will verify my statement.
I may have over simplified my comment regarding bad guesses. I meant to refer to open delta banks that use the smallest transformer in common use by the utility for the second transformer.

A sample problem.
Two 50 KVA 240V transformers in open delta.
The three phase load is 25 KVA resistive.
Find the KW and KVA.
KW will equal KVA, equals 25 KW.
Line current equals 25000VA/sqrt3/240V = 60A
Transformer load; 60A x 240V/1000 = 14.4 KVA
Total KVA equals 14.4KVA x 2 transformers = 28.8KVA
The current is out of phase with the voltage on an open delta bank with a three phase load. Hence the discrepancy between KW and KVA.
And, yes my figure of 86% may be 83% but I need some more coffee before verifying the correct number.
If you work some sample KW vs KVA problems you will soon derive the correct figure with more certainty than I can ever give you.
To complete your understanding of open delta transformer banks, do some vector sketches and look at the voltage-current phase angles and resultant power factors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brj5144]

When reading your replies I don't think I made myself clear. I do not need to calculate the load on the open delta transformer bank. I need to calculate the load on each primary phase. I am trying to balance the phases between three phase switches.

Here is how I am doing it. I have the kilowatt hours used by the transformer bank. From that I calculate KVA for the transformer bank.

Lets say that I have 3 transformer banks between my 3 phase switches and I want to balance the load on them, so I have a list.
Bank - 1
KVA - 100
Type - Single phase on A
Load on Phase A - 100KVA
Load on Phase B - 0KVA
Load on Phase C - 0 KVA

Bank - 2
KVA - 100
Type - Three Phase Symetrical
Load on Phase A - 33.3KVA
Load on Phase B - 33.3KVA
Load on Phase C - 33.3KVA

Bank - 3
KVA - 100
Type - Open Delta 100KVA transformer on B, and a 25KVA trans on C
Load on Phase A - 0KVA
Load on Phase B - ????
Load on Phase C - ????

I am trying to guesstimate the load on the Primary phases of the open delta station. A guy who has done this for years told me that he estimated 80% on the big transformer and 20% on the small transformer.

Which would give me:
Bank - 3
KVA - 100
Type - Open Delta 100KVA transformer on B, and a 25KVA trans on C
Load on Phase A - 0KVA
Load on Phase B - 80% of 100KVA = 80KVA
Load on Phase C - 20% of 100KVA = 20KVA

I could then add them up and get my balance.
A Phase = 133.3 KVA
B Phase = 113.3 KVA
C Phase = 53.3 KVA

I now see I have unbalanced phases between these two three phase switches.

My Question: "is the 80% 20% a good estimate or is there a better way of doing this or is this useless."

Thanks

 

[waross]

Ok I see what you are getting at.
Single phase transformers have single phase loads.That is a no-brainer.
Three phase banks tend to have fairly well balanced three phase loads. No problem.
Open delta banks may have balanced three phase loads, but are more likely to have a mix of single phase and three phase loads. The mix of the loads should be known. It may vary so widely that a rule of thumb may be useless in most systems.
However, most mixed systems have single phase metering and three phase metering. With this information available you shouldn't have to guess.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

Bank - 3
KVA - 100
Type - Open Delta 100KVA transformer on B, and a 25KVA trans on C
Load on Phase A - 0KVA
Load on Phase B - ????
Load on Phase C - ????

You don't have one transformer on B and one on C.

On an open-delta bank, the two transformers are connected to three phases. If the two transformers are the same size, you would assume a balanced three-phase load. If the load is a balanced three-phase load, the load on the primary phases will be balanced. So, if the total load is calculated to be 300 kVA, there would be 100 kVA on each phase.

The tricky part is if the load is unbalanced. Let's say you have a 100 kVA transformer connected between A and B and a 25 kVA transformer connected between B and C. I would assume that the calculated load would be proportioned such that 50/125=40% of it is balanced on the three phases and 75/125=60% is divided between Phases A and B.

 

[davidbeach]

jghrist,

It could be open wye-open delta or it could be open delta-open delta. His 0kVA on phase A assumes an open wye-open delta connection using B and C. You are assuming open delta-open delta. There isn't really enough information to solve the problem.

 

[brj5144]

Thanks Bill.
So you are saying that if I have an open delta station with 3 meters; 1 three phase meter and 2 single phase meters that all of the load on the single phase meters are on the phase with the big transformer and all of the load on the three phase meter is balanced between the two phases.

Example:
Open delta station - 75 kva on B and 25 KVA on C
Meter 1 - Three phase 25KVA
Meter 2 - Single Phase 10KVA
Meter 3 - Single Phase 15KVA

So this station has
A Phase - 0 KVA no transformer or meter is using A phase
B Phase - (1/2 of 25 KVA)load from meter 1 and 10 KVA load from meter 2 and 15 KVA load from meter 3
C Phase - (1/2 of 25 KVA) three phase load from meter 1

So My load is
A - 0 KVA
B - 37.5 KVA
C - 12.5 KVA

Does this sound correct.

Thanks

Am I correct.

 

[stevenal]

kVA cannot be added linearly, you must separately add kW and kVAr.

Even if all load is resistive (unusual for three phase loads), you'll still have reactive current flowing on the primary side. One phase provides leading VArs, the other lagging. This means 80%+20% does not = 100%.

 

[waross]

I was called away before I could finish my previous comments.
I was going to add that an estimate such as you want is very system dependent.
When large residential three phase air conditioning units were more common a typical installation would be a large transformer to supply a group of homes with single phase light and power and a teaser transformer to supply 1 or more 3Ph A/C units.
If your 80/20 split is valid for 1 A/C unit it will not be valid for two A/Cs on a bank.
A rural system may have quite a few open delta banks powering irrigation pumps. Some of these will probably be a large transformer feeding 120/240 to the farm house and a teaser transformer to supply three phase for the pump. In a lot of instances the pump will be remote from the farm house and there will be two equally sized transformers feeding just the pump.
I saw an open delta system considered for a bowling alley in a city. The load was a couple of fractional HP motors on each Automatic Pin Setter. Such a load will be very light and intermittent.
The point is:: There is no one size fits all rule of thumb.
You must understand the system and remember that the load KVA does not equal the KVA loading on the transformers.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[busbar]

This reference may be helpful. Page 4 of…

http://www.6L6.net/busbar/dat/epelectric.dso+section+TD.deltaload.doc